The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutting elements mounted on the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutting elements, at least some of the cutting elements each comprising a preform cutting element having a superhard front cutting face.
The invention is particularly, but not exclusively, applicable to drill bits of the kind in which the cutting elements comprise preforms having a thin facing layer of polycrystalline diamond bonded to a backing layer of tungsten carbide. Such bits and cutting elements are well known and will not therefore be described in detail.
When drilling deep holes in subsurface formations, it often occurs that the drill bit passes through a comparatively soft formation and then strikes a significantly harder formation. Also there may be hard occlusions within a generally soft formation. When a bit using preform cutters meets such a hard formation the cutting elements may be subjected to very rapid wear or damage.
It has therefore been proposed to provide, on the rearward side of at least certain of the preform cutting elements, which may be regarded as primary cutting elements, secondary abrasion elements which are set slightly below (or inwardly of) the primary cutting profile defined by the primary cutting elements.
In this specification, the primary cutting profile is defined to mean a generally smooth notional surface which is swept out by the cutting edges of the primary cutting elements as the bit rotates without axial movement. The secondary profile is similarly defined as the notional surface swept out by the secondary elements.
With such an arrangement, during normal operation of the drill bit the major portion of the cutting or abrading action of the bit is performed by the preform primary cutting elements in the normal manner. However, should a primary cutting element wear rapidly or fracture, so as to be rendered ineffective, for example by striking a harder formation, the associated secondary abrasion element takes over the abrading action of the cutting element, thus permitting continued use of the drill bit. Provided the primary cutting element has not fractured or failed completely, it may resume some cutting or abrading action when the drill bit passes once more into softer formation.
The secondary elements may be formed in a variety of ways. For example, U.S. Pat. Nos. 4,718,505 and 4,889,017 describe a secondary abrasion element comprising a plurality of particles of superhard material, such as natural diamond, embedded in an elongate stud-like carrier element having one end wholly enclosed within a socket in the bit body which is spaced rearwardly from the respective primary cutting elements, and the other end protruding freely from the bit body transverse to the normal direction of rotation of the bit.
Hitherto, it has been the usual practice for all the secondary elements to be set slightly below, or inwardly of, the primary cutting profile by a substantially constant distance, measured perpendicular to the primary profile. However, it is believed that this may be disadvantageous, and may have the effect that secondary elements on some parts of the bit come into operation before secondary elements on other parts, even though they may be subjected to the same local conditions.
Because the drill bit is moving axially as drilling proceeds, the parameter which determines when a secondary element comes into operation, other things being equal, is its position, relative to the primary profile, measured in a direction parallel to the longitudinal axis of rotation of the drill bit (referred to herein, for convenience, as the "vertical" distance). However, the primary cutting profile of the drill bit is usually shaped to provide a "nose" portion which is generally convex, although not necessarily smoothly curved, when viewed in cross-section. The nose portion of the profile is that part thereof which is lowermost when drilling vertically. The nose portion may lie on the central longitudinal axis of the bit in the case where the primary profile is simply convex, or it may comprise an annular area spaced outwardly of said axis in the case where the central portion of the profile is concave, cone-shaped, or otherwise re-entrant.
Due to the generally convex shape of the nose portion, as viewed in cross-section, the vertical distance between each secondary element and the primary profile increases with distance from the nose portion of the profile if the secondary elements are spaced by a constant distance from the profile, measured perpendicularly from the profile.
This means that, when harder formation or occlusions are encountered when drilling, the backing-up or depth stop function is not shared equally between the secondary elements, but falls mainly on the secondary elements nearer the central axis of the bit, leading to excessive wear and/or failure of those elements.
The present invention therefore sets out to provide an improved form of drill bit in which this disadvantage may be alleviated or overcome.